Basic Information

The Necronomicon is a fictional tome of eldritch lore created by H.P. Lovecraft as part of the meta-narrative of his cthulhu mythos, and subsequently cited by various other authors in their own mythos works. Lovecraft's own etymology of the name suggested that it meant "book of the laws of the dead", but this is considered to be bad Greek by other authorities1 who translate it rather as "book of dead names" or "book concerning the dead".

All of those names, ironically, seem completely inappropriate given that it's actual content (never even summarised and varying from author to author) seems to consist mainly of assorted revelations about the underlying reality of the universe, descriptions of various mythos entities and the occasional spell and magical ritual. Traditionally the content overall was meant to be devastating to the reader's sanity.

According to Lovecraft the book was originally written under the title Al-Azif2 by "the mad Arab" Abdul Alhazred3. It was then meant to have been translated into Greek by the fictional Theodorus Philetas and into Latin by the very real Olaus Wormius, followed by an English translation by John Dee. The book was said to have been suppressed repeatedly, but never sucessfully eradicated and various other translations - usually from the Greek rather than the Arabic - written over the years.

Despite its entirely fictional character, the necronomicon has occasionally been mistaken for a genuine book - sometimes as the result of pranks (usually involving the prankster adding it to a library catalogue or offering a copy for online auction), sometimes as the result of stupidity and ignorance (usually when attached to a satanism scare) and once by a genuine occultist (specifically Alestair Crowley - although his own theories of magic were completely unrelated to the mythos). Various people have also muddied the waters by stealing the title for their own publications - of which some have been works of mythos fiction and others completely unrelated.

Sources

Bibliography

1. full source reference

Game and Story Use

Having a tome of eldritch lore which is infamous throughout your campaign setting should add to the verisimilitude - it's better for it to be mentioned than for it to appear, but avoid gratuitous referencing or sending PCs on a fetch quest for it. Ideally your necronomicon-like should be cited in other souces (such as villanous journals) and occasionally mentioned as a great source of evil by sages.

RPG variants include Dungeons and Dragons Book of Vile Darkness, whislt similar things exist in other systems.

Ideally game mechanics should stop this - and equivalent works - from being just another reference work or spellbook. The Call of Cthulu RPG makes studying the Necronomicon roughly as harmful to the sanity as meeting Cthulu himself face-to-face (which is to say "extremely") - with damage decreasing in the event of poor, partial or expurgated translations. Ideally a work of this sort of significance should have equal levels of protection unless you expect your PCs to use it as a set of Cliff Notes for the rest of the campaign.

Note that some mythos stories (for example The Dunwich Horror) do give examples of heroic types dipping into the Necronomicon in search of weapons to use against mythos entities, but they usually pay a high price for such knowledge.

Adaption decay is very much a thing in universe - the closer to the original text, the more useful - and more sanity blasting - the work becomes. Later versions suffer from several iterations of blind guess translation and occasionally expurgation, not to mention incomplete source manuscripts.

Mistaken identity can also be a thing - a real, but elusive work of occult knowledge might spawn outright fakes that serve as chaff for investigators trying to track down the real thing.

Footnotes

1. Which wouldn't necessarily invalidate it - quite a lot of medieval Greek and Latin was very poor stuff by classical standards.

2. Supposed to refer to a noise of nocturnal insects, said to be made by demons, but variously translated as "howling of the wind" or more generally "strange noise"